The enzyme, polo-like kinase (PLK), is a mammalian serine/threonine kinase
involved in cell cycle regulation. A great deal of evidence regarding the role
of PLK in the cell cycle has been obtained through studies of cultured cells,
though little is known about its function or even expression
in vivo. The endometrium undergoes rapid proliferation
and differentiation under ovarian steroid hormone control during the 28-day
cycle. Thus, normal endometrium provides an excellent model in which to study
the hormone dependency of PLK expression. In the present study, we examined
the features of PLK expression in 20 samples of normal human endometrium
during the menstrual cycle. The expression of Ki-67 and proliferating cell
nuclear antigen (PCNA) were also examined as markers of proliferation.
Immunohistochemical studies showed that PLK staining was detected in the
basement membrane of many endometrial glands, stromal cells, and some
endothelial cells. The number of PLK-positive endometrial gland cells was
significantly higher in the late proliferative phase (19.16%
4.98%) and the early secretory phase (19.28% 4.99%) than
in the early proliferative phase (2.60% 2.33%) or the late
secretory phase (5.76% 2.16%)
(P<0.0001). PLK expression seemed to be correlated
with the expression of Ki-67 and PCNA in many endometrial glands and stromal
cells particularly in the late proliferative phase, reflecting a role of PLK
in cellular proliferation. Nevertheless, in the early secretory phase, at
which point the expression of Ki-67 and PCNA decreased in endometrial glands,
PLK was strongly expressed. This finding suggests that PLK may have some
post-mitotic functions in certain specialized cell types. Although the highest
expression of PLK was observed in the late proliferative and the early
secretory phases, the expression drastically decreased in the late secretory
phase. These findings, taken together, indicate that the expression of PLK in
normal endometrium fluctuates over the course of the menstrual cycle,
suggesting in turn that PLK is associated with hormone-dependent cellular
proliferation and that hormone functions may be involved in its regulation.
Immunolocalization of Nuclear Transcription Factors, DAX-1 and COUP-TF II, in the Normal Human Ovary: Correlation with Adrenal 4 Binding Protein/ Steroidogenic Factor-1 Immunolocalization during the Menstrual Cycle